Alberta Bee News
October 2021
Hive-side FAQ: Honey bee viruses By Renata Borba and Emily Olson, Alberta Beekeepers Commission Tech Transfer Program
There are 18 known viruses that afflict honey bees. In strong healthy colonies, viruses at low levels typically do not cause major problems. However, when combined with co-stressors such as low access to adequate food sources, environmental conditions, or other diseases/pests, the effects are exacerbated and can lead to colony mortality.
Virus
Symptoms
Deformed Wing Virus
Adult bees will have shriveled or mis-shaped wings and shortened abdomen.
What are the most common viruses?
Sacbrood Virus
Discoloured larvae (yellow or gray colour) with head slightly raised. May resemble AFB or EFB but when pulled out of the cell, the fluid filled sac remains intact.
The most common viruses we detect in honey bee colonies are Black Queen Cell Virus (BQCV), Sacbrood Virus (SBV), Deformed Wing Virus (DWV), and Varroa destructor Virus (VDV). Chronic Bee Paralysis Virus (CBPV) is less common, but studies have shown that there is a positive correlation between CBPV and colony overwinter mortality.
Varroa destructor Virus More virulent strain of DWV and leads to the premature death of adult bees.
How do bees become infected?
Chronic Bee Paralysis Virus
Infected adults are unable to fly, will cluster together, and crawl on the ground. Wings and bodies will appear to be trembling. Bloated abdomens and dysentery may be observed. Some bees will appear greasy, hairless, and black.
Black Queen Cell Virus
Queen pupae will have a pale yellow sac, similar to sacbrood. Once the pupae dies, the walls of the queen cell will turn black. Symptoms have also been observed in drone brood. BQCV may be associated with k-wing and Nosema disease.
Bees can become infected by viruses via 2 transmission routes: horizontal and vertical transmission. Horizontal transmission consists of transmission of viruses among individuals of the same generation (e.g., worker-worker and worker-queen) by topical contact. This includes contact with contaminated feces (when cleaning the colony or foraging on contaminated flowers), contaminated food (mouth-to-mouth food sharing among workers and when tending the queen), venereal transmission (from drones to queens during nuptial flights), and via vector-mediated transmission (e.g., Varroa mites). Varroa mites are a major mechanical vector of several honey bee viruses. In other words, mites can acquire viruses when feeding on the fat body or hemolymph (‘bee blood’) of infected bees and transmit these viruses to a healthy bee by feeding on them shortly after. There is also abundant evidence that mites are an effective biological vector to viruses, since some beeviruses are able to multiply inside Varroa mites. For example, DWV and VDV can be transmitted by mites during feeding activities from an unhealthy bee to a healthy bee, and also replicate/multiply while inside the mite. In this case, the mite functions as both a mechanical vector and a biological vector to DWV and VDV, which is the reason why Varroa mites are considered the most damaging parasite to honey bee colonies. Varroa mites also weaken bees’ immune systems, making them more susceptible to infections. Vertical transmission occurs when viruses are transmitted to the next generation, from queens to their eggs. When queens become infected with viruses, they can become a source of transmission and spread within the colony to the next generation of bees.
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